If ZMusic is to act like an external library it may not call delete on external objects because there is no guarantee that they use the same allocator. Deletion must be done as a virtual function to ensure that the correct operator delete gets called, which, unlike the actual destructor is not virtual itself.
Now there is only one single entry point for both, instead of previously 2 entry and 4 exit points.
This also eliminates the explicit shutdown of ZMusic. Timidity++'s two buffers have been put in containers that self-destruct on shutdown and calling dumb_exit is not necessary because the only feature requiring it is not used by any code in the music library.
This was getting a bit unwieldy. The include path setup is not perfect yet, that's work for later.
(It's about time we're getting C++20 with modules so that this include path madness can be put to an end.)
The organization here is now the same as for the Timidity++ device, i.e. it is the device owning the instruments to give better control over their lifecycle.
